European Union leaders, meeting in Brussels, insisted on "the urgent need for Russia to de-escalate tensions caused by the military build-up along its border with Ukraine and aggressive rhetoric".

Separately, NATO's members used similar language, rejecting "the false Russian claims of Ukrainian and NATO provocations" and urged Moscow to "immediately de-escalate, pursue diplomatic channels, and abide by its international commitments."

Both organisations, which share most of their member states, reiterated a threat to impose "massive consequences" on Moscow through sanctions, coordinated between European powers and Washington.

On Thursday, the EU leaders agreed to renew for six months existing economic sanctions imposed on Russia in 2014 in response to its annexation of Crimea, a Ukrainian region.

The US has used similar language of "massive" retaliation in the event of a Russian invasion while attempting to reach out to Moscow to defuse the situation.

NATO Secretary General Jens Stoltenberg branded Russia the "aggressor".

Russian President Vladimir Putin wants direct dialogue with US counterpart Joe Biden to resolve the stand-off, and is seeking security guarantees to stand down his troops.

But the European leaders in their summit pushed for a return to the "Normandy format", a four-way dialogue between Paris, Berlin, Kiev and Moscow designed to enforce the 2015 Minsk accords, a blueprint for a political settlement.

France's President Emmanuel Macron said the goal was to "re-engage Russia in the only political framework that can resolve the Ukrainian situation".

This, Germany's Chancellor Olaf Scholz admitted, is "not an easy thing, we should have no illusions."

On Wednesday, Russia handed a list of security demands to US Assistant Secretary of State Karen Donfried, who then came to NATO headquarters in Brussels to discuss them with Stoltenberg.

Afterwards, the NATO members said any dialogue would have to "take place in consultation with NATO's European Partners".

The NATO chief also met Ukraine's President Volodymyr Zelensky, and gave a joint news conference to insist that any decision on membership was a matter for Kiev and the alliance's 30 member states.

"We will not compromise on the right of Ukraine to choose its own path. We will not compromise on the right for NATO to protect and defend all NATO allies," Stoltenberg said.

- No compromise -

He said there would also be no compromising on NATO's partnership with Ukraine, describing it as important for both sides and "not in any way a threat to Russia".

On Wednesday, Zelensky attended a prior summit with EU leaders and said that most of them supported Ukraine's position in the conflict.

But he is frustrated that European powers in particular have refused to take preventive action against Russia, preferring to threaten a response in the event of Russian aggression.

"Since 2014, since the start of the war, I believe that basically Russia pushed Ukraine into NATO," he said.

"Basically I believe that today Russia itself is paving the difficult path of Ukraine to NATO."

He complained that, in his view, some EU members did not seem to have understood the extent of Ukraine's peril and urged them to act swiftly.

On Wednesday, Russia's Deputy Foreign Minister Sergei Ryabkov told Donfried that NATO should halt its eastward expansion and withdraw a promise that Ukraine could become a candidate for membership.

- Nothing yet agreed -

A US statement said Donfried would stress "we can make diplomatic progress on ending the conflict in the Donbas through implementation of the Minsk agreements in support of the Normandy Format".

Ukraine and its closest supporters in the West want to cancel the opening of the Nord Stream 2 pipeline, which will carry Russian gas supplies to Germany, bypassing Ukraine.

Germany's new leader has been cautious on this, and some of the leaders meeting in Brussels fear pre-emptive sanctions will provoke rather than deter Russia.

Scholz said the pipeline was a private-sector project and that an apolitical approval process under EU energy law is underway. The EU will ensure that Ukraine's "integrity is not violated" by the decision, he said.

Getting Russia back to the negotiating table will not be easy.

"Moscow does not want to return to the Normandy format and wants to negotiate with the United States," a senior European diplomat told AFP.

The diplomatic track was established at a summit in Minsk in 2015 where Putin accepted that France and Germany should play the role of moderators in the talks between the two belligerents.

"The Americans support a return to this format, and talks are underway with Moscow, but nothing has been agreed yet," the European envoy said.